1. Field of the Invention
This invention relates to a vibration ram used for tamping or compacting the ground in road building and other construction works.
2. Description of the Prior Art
There are known in the art vibration rams of the type in which an inner cylinder is slidably inserted into an outer cylinder. The inner cylinder having inserted thereinto a piston rod connected through a connecting member to a connecting rod hinged to a crank to which the rotative force of an engine is transmitted through a gearing mechanism or a belt pulley assembly. Coil springs compressed with a predetermined compression force are interposed between the piston secured to an end of the piston rod and the top and bottom plates of the inner cylinder such that the upper and lower coil springs will be alternately compressed by the piston in accordance with the reciprocating movement of the engine-driven piston rod. The inner cylinder is vibrated by the repulsive force of the compressed coils springs to accordingly move a foot plate secured to the lower end of the inner cylinder to thereby effect the desired tamping of the ground.
In these known types of rams, however, since coil spring units are provided on both the upper and lower sides of the piston, it is necessary to elongate both the inner and outer cylinders by an amount corresponding to the height of the two coil spring units. This results in both an elevated position of the center of mass of the ram device and an increase in the weight thereof. This increased height and weight requires a great deal of extra labor and inconvenience to the ram operator. Referring to Fig. 1, supposing that the center of mass of the ram is G, the position at which the operator grips the operating handle for supporting the ram is A and the end point of the foot plate is B; then the ram has a moment of W(x.sub.1) urging it to rotate counterclockwise about the point B when the foot plate impacts against the ground surface. In order to resist this turning force, the operator is required to apply a force P to the ram equal to (x.sub.1 /x.sub.2)W. Thus, the smaller the value of P, the less labor is required by the operator; but since the distance x.sub.1 is increased when the center of mass G is high, the value of P is accordingly increased.
The problem becomes more severe because the ground to be tamped by a ram is usually rugged and of irregular solidness; and further, the area of the foot plate portion engaged with the ground surface is kept small to increase the tamping force applied per unit of area relative to the size of the ram body. Further, the ram body has a tilted low-stability structure for providing a tendency to advance in a forward direction while in operation. Further, the high center of gravity makes the device not only unstable both in the rearward and force-applying directions as described above, but also tends to turn over in all other directions, so that the operator is required to exercise a great deal of effort to prevent such overturn.
Thus, for the above-discussed reasons, lowering of the center of mass and reduction in wieght have been important objects for improvement in the field of rams.